You can’t tell how fast Saturn is spinning by watching the clouds swirling at its surface. But ripples in its rings reveal how fast the planet rotates: Its day flies by in 10 hours, 33 minutes and 38 seconds.

“That’s a really fast clip,” says astronomer Christopher Mankovich of the University of California, Santa Cruz, who reports the rotation rate in the Astrophysical Journal on January 17. Saturn, with a radius of about 58,000 kilometers, is about nine times the size of Earth yet its day is less than half as long.

Scientists previously estimated possible lengths for a Saturnian day using radio measurements from the Voyager spacecraft in the 1980s and from the Cassini orbiter in the 2000s. But those estimates, while in the same ballpark as the new one, varied by about 20 minutes.

In 1993, astronomers Mark Marley and Carolyn Porco, who later led Cassini’s imaging team, realized that Saturn’s seismic activity changes the planet’s gravity enough to make its rings of tiny, orbiting particles shift in response. Because rotation stirs up the planet’s gas, the frequency of those inner oscillations could be used to figure out how fast the planet spins. Cassini’s 2004 arrival in Saturn’s orbit finally delivered ring images good enough to test the idea.

Using those images, snapped up to the 2017 end of the NASA orbiter’s mission (SN Online: 9/15/17), Mankovich and colleagues measured waves in the rings as the rings responded to Saturn’s vibrations. That let the team calculate precisely how fast the planet was rotating beneath the clouds.

“The amount we’ve learned about Saturn’s interior from the rings, of all places, is beyond almost anyone’s wildest imagination,” says Mankovich, whose research team included Marley, at NASA’s Ames Research Center in Moffett Field, Calif.

Knowing Saturn’s rotation rate can help scientists figure out how the gas giant is structured. Astronomers think Saturn has a solid core about 15 times the mass of all of Earth, topped by layers of different phases of fluid hydrogen. Mankovich and his colleagues assumed Saturn rotates as a rigid ball, but it could instead be layered like an onion, with each layer rotating at a different speed, Mankovich notes. The team’s next step is to peel back the onion layers.